Lithographic printing is a printing system in which printing is conducted using a printing plate with ink-receptive image areas and hydrophilic ink-repellant non-image areas. Namely, printing is conducted in such a manner that a dampening solution is applied to the surface of the printing plate. The solution is retained in the hydrophilic areas, but repelled by the ink-receptive image areas to which ink is to be applied. It is important for the ink and the dampening solution to be applied to the surface of the printing plate with a good ink-water balance to achieve successful printing. When the amount of the dampening solution applied to the surface of the plate is too large, ink is intensively emulsified and offset or failure in drying occurs. When the amount of the dampening solution is too small, ink adheres to the non-image areas and scumming occurs.
For better control of the balance between ink and the dampening solution, dampening solutions usually contain, in addition to water, about 10% by weight of isopropyl alcohol (IPA) to reduce surface tension, various hydrophilic materials such as gum arabic, carboxymethyl cellulose (CMC), citric acid and various surfactants, an acid such as phosphoric acid as an affinitizing agent to remove oxides on the surface of the plate and ammonium bichromate or nitrates as a corrosion inhibitor for the plate. When the dampening solutions containing IPA are used, good prints can be obtained, transfer of the dampening solution from a pan is good and as a result, water tolerance is increased and workability is improved. The reason of these advantages can be assumed as follows: (1) the wetting of the hydrophilic non-image areas of the plate becomes good and (2) the wetting of dampening rollers by the dampening solution is good and as a result, transfer of water from a pan to the form rollers and the plate is good. However, IPA is a flammable hazardous material. Hence, fire hazards are a concern. Further, IPA is harmful to the human body. Accordingly, a lithographic dampening solution without IPA has been desired.
To this end, JP-B-55-19757 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses dampening solutions for lithographic printing which contain polyalkylene oxide alkyl ether surfactants, JP-B-46-30323 (corresponding to U.S. Pat. No. 3,547,632) discloses dampening solutions for lithographic printing which contain polyglycol and an anionic surfactant, and JP-A-63-25093 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") (corresponding to EP-A-251621) discloses dampening solutions for lithographic printing which contain polyethylene oxide surfactants.
The surface tension can be certainly reduced by adding usually 0.1 to 0.5% by weight, based on the total weight of the solution, of the surfactant. However, the transfer of the dampening solutions from a pan is poor in comparison with those containing IPA and the wetting of the hydrophilic non-image areas of the plate is not satisfactory.